If your display is not registering any speed your speed sensor may not be properly aligned. Follow this guide to align it.
The speed sensor consists of a sensor attached to the non drive side chain stay on the bike which connects to the motor and a magnet on a wheel spoke. The rotation of this magnet is how the speed of your bike is determined by the controller.
If the magnet is mis-aligned you won't get any speed reading. This is very easy to fix however and just involved moving the magnet.
You need this magnet to be on a non-drive side spoke of the rear wheel. To loosen this magnet for adjustment you will need a screwdriver with a T20 Torx bit.
Loosen the screw then move the magnet to the correct position- see the images below for correct positioning.
When the magnet is in the correct position the speed sensor should deliver the bike's proper speed to the display.